Most of the people terrible at estimating size, be they men or women. Outside of this thread and some other places like r/measuredpornstars i don't see many guys who are good at correctly estimating length or girth. On AdultDVDTalk and MattersOfSize, places with mostly male userbase, one can easily find how Julio and Tony Duncan actually at least 9.5", everything under 7" is small and 6" girth is almost average. I'm sure women are pretty much the same when it comes to estimating sizes.
By the way, it seems i failed at our little argumet regarding Justin Slayer. I just can't find any good shots with him and other guys where are their dicks parallel to each other and on equal distance from camera. The best one i found with Billy Glide and even this is not very useful:
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I'll try to find if he has any comparisons to objects like bottles, pens, tools or something like that. So far no luck with this.
That doesn't mean that i agree with your estimation of him at 7.25" btw., just that i can't find good comparisons of him to other guys.